1. Field of the Invention
The present invention relates to a method of exciting a light for an optical amplification medium fiber which is used in industrial area such as information communication, a laser medical operation, or a laser manufacturing operation; a structure for emitting an excited light into an optical amplification medium fiber; an optical fiber amplifier; and an optical fiber laser.
2. Description of Related Art
For a conventional type of the optical amplification medium fiber as shown in FIG. 12, there has been an optical amplification medium fiber which is provided with an inner cladding 102 around a core 101 into which a rare-earth element is doped, an outer cladding 104 which is disposed on an outer periphery of the inner cladding 102 via a porous layer 103, and a coating 105 thereon. In the porous layer 103, holes 106 and a connecting section 107 which connects the inner cladding 102 and the outer cladding 104 are formed alternately in a circular direction. (See “Jacketed air clad cladding pumped ytterbium-doped fiber laser with wide tuning range” by J. K. Sahu et al, Electronics Letters, Great Britain, 2001, Vol. 37, pages 1116–1117)
There is a large relative refractive index difference between the inner cladding 102 and the porous layer 103 in such an optical amplification medium fiber 110 which has a porous fiber structure. Therefore, it is possible to emit an excited light which has a large aperture number to be incident into the optical amplification medium fiber 110. Thus, there is an advantage in that it is possible to form an optical fiber laser which has a great output. If it is intended to emit the excited light into a side surface of the optical amplification medium fiber 110 so as to excite the optical amplification medium fiber optically, the excited light is dispersed by the porous layer 103. Therefore, efficient incidence cannot be realized; therefore, it is a common procedure in that the excited light is emitted into an end surface of the optical amplification medium fiber 110.
FIG. 13 is a general view for an example for a conventional method for exciting a optical amplification medium fiber optically. In such a method for exciting a light, an excited light 124 is emitted to be incident into the inner cladding 102 of the optical amplification medium fiber 110 by condensing a light of the excited light 124 which is emitted from a laser diode (source for an excited light) 126 on an end surface 111 of the optical amplification medium fiber 110 by a light condensing lens 127 so as to excite the rare-earth element which is doped into core 101 of the optical amplification medium fiber 110.
FIG. 14 is a general view for a first example for a conventional optical fiber amplifier. In such an optical fiber amplifier 140A, a signal light 143 which is output from an end surface 142 of an optical fiber 141 for incident signal light is condensed by the light condensing lens 144 so as to be incident into an end surface 111 of the optical amplification medium fiber 110 through a optical filter 145. Also, the excited light 124 is condensed by the light condensing lens 127 and reflected by the optical filter 145 so as to be incident into the end surface 111 of the optical amplification medium fiber 110. In the optical amplification medium fiber 110, the signal light is amplified by a rare-earth element which is excited by absorbing the excited light 124 so as to be output from an end surface 112 which is disposed opposite to the optical amplification medium fiber 110 as an output signal light 146.
FIG. 15 is a general view for a second example for a conventional optical fiber amplifier. An optical fiber amplifier 140B is provided with a plurality of laser diodes (source for excited light) 126, so as to enhance the intensity for the excited light 124 which is emitted to be incident into the optical amplification medium fiber 110. Consequently, the excited light 124 which is output from these excited light sources 126, 126 are mixed by an optical mixing element 128 which is formed by a silica glass member so as to emit the excited light 124 into an end surface 111 of the optical amplification medium fiber 110 via a light condensing lens 127 and an optical filter 145. By doing this, the optical amplifier 140B can realize more efficient amplitude than in a case in which a piece of excited light source.
FIG. 16 is a general view for a third example for a conventional optical fiber amplifier. In the optical fiber amplifier 140B, a plurality of optical amplifying medium fibers 110a, 110b, 110c are connected via light condensing lenses 144 and optical filters 145 for mixing the excited light 124 and the signal light 143, 147. That is, the signal light 143 which is output from an end surface 142 of the optical fiber 141 into which a signal light is emitted to be incident is emitted to be incident into a first optical amplification medium fiber 110a, a second optical amplification medium fiber 110b, and a third optical amplification medium fiber 110c successively and amplified. The signal light under condition of a space beam 147 is transmitted in a space among the optical amplification medium fibers 110a, 110b, and 110c so as to be mixed with the excited light; thus, the signal light is emitted to be incident into next optical amplification medium fibers 110b and 110c. By doing this, it is possible to obtain an output signal light 146 which has a greater intensity.
FIG. 17 is a general view for another example for a conventional method for exciting a optical amplification medium fiber optically. A so-called double cladding optical amplification medium fiber 210 which is used in this method is proyided with a core 201, an inner cladding 202 which is formed by a silica glass member so as to be formed on an periphery of the core 201, and an outer cladding 203 which is formed on an outer periphery of the inner cladding 202. The outer cladding 203 is a resin coating of which refractive index is slightly lower than that in the inner cladding 202 (for example, the refractive index in the inner cladding 202 is 1.45 and the refractive index in the outer cladding 203 is 1.42). It is possible to realize a waveguide function in the inner cladding 202 by using such refractive index difference.
In this method for exciting the light, an outer cladding 203 of the optical amplification medium fiber 210 is removed in a part of a longitudinal direction. Furthermore, a part of an outer periphery of the inner cladding 202 which is exposed in a coating removed section 211 is ground so as to form a planar ground section 207. An end surface 208 of the optical fiber 220 for excited light incidence is cut diagonally so as to be connected to the ground section 207. Furthermore, a protective resin layer 214 is disposed so as to protect the connecting section 213 and the ground section 207.
According to such a method for exciting the light, the excited light 224 which is transmitted in the optical fiber 220 for excited light incidence is emitted from the connecting section 213 so as to be incident into the optical amplification medium fiber 210 for excited light incidence. Thus, a light is excited in the optical amplification medium fiber 210. (See English Specification for U.S. Pat. No. 6,370,297)
In the optical fiber amplifiers 140A to 140C which use the optical amplification medium fiber 110 which has the above porous fiber structure, it is necessary to emit the signal light 143, 147 and the excited light 124 so as to be incident into the optical amplification medium fiber 110 as a space beam; therefore, it is necessary to dispose an optical system such as a light condensing lens 144 and an optical filter 145. In addition, it is necessary to dispose the optical fiber 141 into which the signal light is emitted to be incident and the excited light source 126 in aligned condition simultaneously. Therefore, very delicate operations such as adjusting an alignment for an optical axis of a light is necessary. Also, even though the optical axis is adjusted desirably, there are problems in that an optical axis may be dis-aligned by a mechanical factors such as a vibration and collision or by a temperature condition; thus, a desirable amplification function may be disturbed.
Also, in a case in which a double-cladding optical amplification medium fiber 210 is used, there is a slight difference for the refractive index for the inner cladding 202 and the outer cladding 203. Therefore, only a component which has a small incidence angle θ in the excited light 224 which is emitted so as to be incident into the optical fiber 220 for excited light incidence can be transmitted in the optical amplification medium fiber 210. A component which has a large incidence angle θ in the excited light 224 leaks in a condition of light loss out of the optical fiber 210 via the outer cladding 203 from the inner cladding 202. That is, there is a problem in that efficiency for emitting the excited light 224 to be incident into the optical amplification medium fiber 210 is not enough; thus, the light loss is undesirably great.
Also, in recent years, optical fiber laser and amplifier have been studied which use a laser oscillation medium such as a silica glass member to which a rare-earth element such as erbium (Er), neodymium (Nd), ytterbium (Yb), and holmium (Ho) is doped and a host glass such as a fluoride glass member.
There is an advantage for an optical fiber laser in that it is possible to realize a small device highly efficiently such that the laser oscillation medium serve for a transmitting medium compatibly. The optical fiber laser is used for various industrial use such as an optical communication, an optical sensor, a material science, and medical science according to the above feature. In particular, a laser which has a higher output power is longed for industrial use such as an optical communication and a material science.
The realization of the higher output power depends on an introduction of the excited light into an area (normally a core is named for such an area) into which a laser activating medium (rare-earth ion) is doped in the optical fiber laser.
An area to which a laser activating medium is doped such as an outer diameter of the core may be approximately 10 μm for an optical fiber laser which is manufactured according to a common method in which the excited light is introduced into an end surface of the rare-earth doped fiber. Therefore, it is very difficult to introduce an excited light into the core efficiently. Also, if there is a dust on an end surface into which the excited light is emitted to be incident in the rare-earth-doped fiber, there is a concern that the end surface may be damaged by a heat.
A method is proposed for introducing the excited light into the core efficiently in which a double-cladding fiber is provided with a first cladding and a second cladding for the laser medium so as to introduce the excited light into an end surface of the double cladding fiber or the side of the double cladding fiber.
An optical fiber laser shown, for example, in FIG. 22 is used for a method in which the excited light is introduced into an end surface of the double cladding fiber.
In such an optical fiber laser, the excited light which is emitted from the excited laser diode module (hereinafter called an “excited LD module”) 201 is condensed by the light condensing lenses 202, 203 so as to be introduced into an end surface of a rare-earth-doped optical fiber 205 via a resonator mirror 204; thus, a light which is emitted therefrom resonates in the resonator mirror 204. Accordingly, the resonating excited mirror is emitted from another end surface of the rare-earth-doped fiber 205 so as to be emitted thereoutside in a laser light condition via the resonator mirror 206.
The cross section of the first cladding of the rare-earth-doped fiber 205 is larger than a core in a non-double-cladding optical fiber; therefore, it is possible to introduce more amount of the excited light into the fiber. Therefore, it is possible to realize a higher output power in the optical fiber laser.
In such a method for introducing the excited light into a side of the double-cladding fiber, for example, a plurality of optical fibers are bundled and the excited light is emitted from a periphery of a structure which is formed by optical medium unitarily. By doing this, an optical fiber laser for emitting the laser light from the end surface of the optical fiber is formed (See Japanese Unexamined Patent Application, First Publication No. Hei 10-190097).
The excited light is introduced into such an optical fiber laser from the side of a plurality of the optical fibers; therefore, an area into which the excited light is introduced can be enlarged greatly.
However, an intense excited light is still condensed on an end surface of the rare-earth-doped fiber according to a method in which the excited light is introduced into an end surface of the double-cladding fiber. Therefore, there is a concern that a section into which the excited light is introduced may be damaged by heat in the double-cladding fiber. If the section into which the excited light is introduced is damaged by a heat, the excited light is not introduced into the rare-earth-doped fiber; thus, the oscillation of the excited light stops in the rare-earth-doped fiber. Therefore, there is a problem that the laser light is not output.
On the other hand, the excited light is transmitted so as to cross a plurality of the optical fibers according to a method in which the excited light is introduced into a side of the double-cladding fiber. Therefore, a problem such as a diminished transmission of the excited light and a dispersion loss may occur in gaps among the optical fibers.
The optical fibers are embedded in an organic bonding agent in order to prevent such a diminished transmission and a dispersion loss. However, such an organic bonding agent has only a low optical power resistance. If the intensity of the excited light is enhanced for obtaining a higher output of the laser light, the organic bonding agent may be denatured; thus, it is not possible to realize a desirable transparency for the organic bonding agent. Accordingly, there may be a case in which the output power of the laser light be decrease extremely. Also, an entire structure which is formed by a plurality of the bundled optical fibers may be damaged by a heat; thus, there may be a case in which the laser light may not be output.